The present invention relates to an image sensor board as a component for various kinds of image sensors such as a flat-bed type image sensor. Further, the present invention relates to an image sensor incorporating such an image sensor board.
A typical prior art contact-type image sensor has a structure shown in FIG. 14. The illustrated image sensor comprises a light source 2e, a casing 4e for accommodating the light source 2e, an image sensor board 6e mounted at the bottom of the casing 4e, a document supporting panel 70e mounted in an upper portion of the casing 4e, a focus lens unit 51e disposed between the document supporting panel 70e and the image sensor board 6e and a platen 90e for transmitting a document K held in contact with the document supporting panel 70e. The image sensor board 6e is provided with a plurality of light receiving elements 52e disposed in an array extending in the primary scanning direction. The document supporting panel 70e is made of a transparent glass material and is held in contact with the platen 90e at a portion, which serves as an image-reading line P.
In reading an image of the document, the light source 2e emits light toward the image-reading line P while the platen 90e transmits the document K in the secondary scanning direction. The light reflected by the document K is focused on the array of light receiving elements 52e by the focus lens unit 51e. As a result, each of the light receiving elements 52e generates an image signal corresponding to the received amount of light. The image sensor board 6e may be, for example, a white plate made of a ceramic material. The image sensor board is disposed so as to close a bottom opening of the casing 4e so that disturbing external light is prevented from entering the inner space 46e located below the focus lens 51e. 
However, with such a conventional image sensor, depending on the conditions of the use, noises are included in reading out the image of the document, which leads to degradation of the quality of the obtained image. The inventors have experimentally found the following to be one cause.
The image sensor board 6e, though having a white surface, does not reflect all the received light, but allows a certain amount of light to pass through the image sensor board. Accordingly, if another light source exists outside the casing 4e, a certain amount of light (including visible rays as well as ultraviolet rays and infrared rays) emitted from such an outer light source passes through the image sensor board 6e to enter the inner space 46e of the casing 4e.
On the other hand, the light receiving element 52e, which may be a phototransistor capable of photoelectric conversion, generates a signal upon receiving visible rays as well as ultraviolet rays and infrared rays. Thus, in the prior art, the light entering the inner space 46 through the image sensor board 6e is unfavorably detected by the light receiving element 52e, which generates an improper signal, thereby causing noises.
This problem occurs even when the image sensor board 6e is made of a synthetic resin such as glass-fiber-reinforced epoxy resin instead of a ceramic material.
It is, therefore, an object of the present invention to prevent disturbing external light from passing through an image sensor board to be received by light receiving elements, thereby enhancing the image reading quality provided by an image sensor.
In accordance with a first aspect of the present invention, there is provided an image sensor board comprising a substrate having an obverse surface provided with a plurality of light receiving elements for generating image signals upon receiving light, and a light shielding layer for covering a reverse surface of the substrate at least entirely over a portion of the obverse surface where the plurality of light receiving elements are mounted.
According to a preferred embodiment of the present invention, the light shielding layer may be black. Specifically, it is advantageous that the shielding layer comprises a black glass material containing a first Fexe2x80x94Crxe2x80x94Co based black pigment and a second Cuxe2x80x94Cr based black pigment in a total amount of 13-25 wt %.
The light shielding layer may cover the entire reverse surface of the substrate. Moreover, the light shielding layer may further extend beyond the substrate.
According to another preferred embodiment of the present invention, the light shielding layer may comprise a light-shielding tape bonded to the reverse surface of the substrate.
According to still another preferred embodiment of the present invention, the light shielding layer may be formed by applying, to the reverse surface of the substrate, a coating material containing a black pigment.
In accordance with a second aspect of the present invention, there is provided an image sensor comprising: a casing; a light source disposed in the casing for emitting light toward an object to be read; an image sensor board having on an obverse surface provided with a plurality of light receiving elements for generating image signals upon receiving light reflected by the object, the image sensor board being disposed so that the obverse surface thereof directed inwardly of the casing; wherein the substrate is covered with a light shielding layer for covering a reverse surface of the substrate at least entirely over a portion of the obverse surface where the plurality of light receiving elements are mounted.